1. Field of the Invention
This invention relates to a non-magnetic toner for developing an electrostatic image, used in electrophotography, and a process for producing non-magnetic toner particles.
2. Related Background Art
Developers used in electrophotographic processes are hitherto commonly produced by a pulverization process comprising melt-kneading a binder resin such as polyester resin, styrene-acrylic resin or epoxy resin and added thereto a colorant, a charge control agent and a release agent, to uniformly disperse them, and thereafter pulverizing the kneaded product into particles with a stated size, further followed by removal of excess fine particles and coarse particles by means of a classifier. However, with a recent trend toward higher image quality, it has become necessary to make toner have much smaller particle diameter.
As toners come to have a weight average particle diameter of 7 .mu.m or smaller as measured by a Coulter counter, there is a tendency that it becomes very difficult to achieve uniform dispersion of materials used and highly efficient pulverization, which have hitherto not come into question, and also to classify toner particles in a sharp particle size distribution.
In order to overcome such problems on toners produced by pulverization, Japanese Patent Publication No. 36-10231, No. 43-10799 and No. 51-14895 propose a process for producing toner particles by suspension polymerization. The suspension polymerization is a process in which polymerizable monomers, a colorant and a polymerization initiator, optionally together with a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a monomer composition, and thereafter this monomer composition is dispersed in a continuous phase such as an aqueous phase, containing a dispersion stabilizer, by the use of a suitable dispersion machine to simultaneously carry out polymerization reaction to obtain toner particles with the desired particle diameters.
This production process does not have the step of pulverization, and hence it is unnecessary to impart brittleness to toner particles and also it is possible to use a low-softening substance in a large quantity, which has been difficult to use in conventional pulverization processes. Accordingly, materials can be selected over a broader range. The process recently attracts notice because of its characteristic features that release agents and colorants, which are hydrophobic materials, may become exposed to toner particle surfaces with difficulty and hence may less contaminate toner carrying members, photosensitive members, transfer rollers and fixing assemblies.
In addition, in recent years, digital full-color copying machines and printers have been put into practical use, so that it has become necessary for toners to be more improved in their performances such as image fidelity, releasability and color reproduction.
As quality requirements for the achievement of image fidelity, in the digital full-color copying machines, toners must be transferred from the photosensitive member to a transfer medium in a larger quantity than in monochrome copying machines. Also, it is foreseen that toners are sought to be made to have finer particle diameters corresponding to finer dots so as to cope with a continuing demand for higher image quality. From this viewpoint too, the polymerization process that can relatively easily produce toner particles having a sharp particle size distribution and fine particle diameter has superior features.
However, the production of toner particles by such a polymerization process has caused many problems when carbon black is used as a colorant.
In the first place, carbon black has on its surface a functional group such as a quinone group that inhibits the polymerizability of polymerizable monomers. Hence, the rate of polymerization decreases to make it difficult to enhance the degree of polymerization, so that the particles may become unstable at the time of granulation to cause agglomeration and coalescence, making it difficult to take out the product as particles.
Secondly, when carbon black is dispersed in polymerizable monomers, the carbon black can be dispersed with great difficulty because it has smaller primary particle diameter and larger specific surface area than other pigments and also has a long structure. Thus, it tends to localize in particles or to cause particles containing no carbon black.
Thirdly, since carbon black has a conductivity, electric charges on the toner particle surfaces tend to leak to tend to cause problems such as fog and toner scatter at the time of development.
To solve these problems, e.g., to cope with the inhibition of polymerizability, there is a method in which carbon black whose particle surfaces have been grafted is used, as disclosed in Japanese Patent Application Laid-open No. 56-116044, and a method in which carbon black whose particle surfaces have been treated with an aluminum coupling agent is used, as disclosed in Japanese Patent Application Laid-open No. 63-210849. These methods, however, require cumbersome steps for the surface treatment of carbon black, take a much time, result in a production cost increase, and are difficult to employ in an industrial scale.
With regard to dispersibility, Japanese Patent Applications Laid-open No. 64-35457 and No. 1-145664 disclose a method by which the dispersibility is improved using a specific dispersant, which, however, is in such a state that can not be said to have been well settled.